1. Field of the Invention
The present invention relates to an optical sampler that produces a low speed signal based on a high speed electric signal so that the high speed electric signal can be analyzed using the low speed electric signal.
2. Description of the Related Art
FIG. 1 schematically shows the structure of a typical optical sampler. In the figure, the signal source 1000 generates: a signal Vin, an electric signal changing at high speed, which is measured by the optical sampler. The optical sampler has an optical modulator 2000, a laser pulse source 3000, a polarizer 4000, an analyzer 5000, an optical detector 6000, a detecting circuit 7000, ad a holding circuit 6000. The optical modulator 2000 is of a bulk type, including, for example, an electrooptical effect crystal 2100, which can be made from LiNbO.sub.3, and a pair of electrodes 2200A and 2200B opposing to each other.
FIG. 2A shows a waveform of an optical pulse PF output by the laser pulse source 3000, FIG. 2B shows a waveform of the signal Vin output by the signal source 1000, and FIG. 2C shows a waveform of the low speed signal LO output by the holding circuit 8000.
The optical pulse PF provided by the laser pulse source 3000 is polarized by the polarizer 4000, which feeds a polarized optical signal Pin to the optical modulator 2000. The sampling frequency of the optical pulse PF is set to be higher than the signal frequency of the signal Vin. Meanwhile, the signal Vin generated by the signal source 1000 is fed to the optical modulator 2000, whereby an electric field is applied to the electrooptical effect crystal 2100 using the pair of electrodes 2200A and 2200B.
The direction of the electric field is right-angled to the direction in which the optical signal Pin advances. Hence, by providing the polarization plane electric field to the electrooptical effect crystal 2100, the polarization plane of the optical signal Pin advancing through the electrooptical effect crystal 2100 is rotated according to the electric field. In other words, the angle of the polarization plane of the optical signal Pin is defined by the electric field, that is to say, by the signal Vin. In this way, the signal Vin is sampled using the polarized optical signal Pin, that is, the polarized optical signal Pin is modulated by the signal Vin. After being rotated, the optical signal Pin is input to the analyzer 5000 which subsequently outputs an optical signal Pout.
FIG. 3 shows a relationship between the signal Vin and the a ratio of the signal Pin to the signal Pout. The relationship is represented by a sine curve. In FIG. 1, it is assumed that the polarizer 4000 and the analyzer 5000 are set so that the difference between the angle of the polarization plane of the polarizer 4000 and that of the analyzer 5000 is 45 degrees. If there is no an electric field, that is to say, there is no signal Vin, a signal Pout corresponding to Vin=0 is fed from the analyzer 5000.
In FIG. 3, the characteristic near Vin=0 is as follows. If the signal Vin is positively applied, the polarization plane of the optical signal Pin is rotated clockwise by +.theta.. Thus, the optical signal Pout is increased according to the sine curve. For example, an optical signal Pout corresponding to Vin=Vp is output. On the contrary, if the signal Vin is negatively applied, the polarization plane of the optical signal Pin is rotated counterclockwise by .theta. (clockwise by -.theta.) . Thus, the optical signal Pout is decreased according to the sine curve. For example, the optical signal Pout corresponding to Vin=Vn is output. In summary, the ratio of the optical signal Pin to the optical signal Pout depends upon the angle of the polarization plans given by the optical signal Pin, and therefore depends upon the signal Vin.
The optical signal Pout fed by the analyzer 5000 undergoes optical/electric conversion in the optical detector 6000, whereby an electric signal corresponding to the optical signal Pout is produced and fed into the detecting circuit 7000. After receiving the electric signal, the detecting circuit 7000 amplifies it to output the amplified electric signal to the holding circuit 8000. The holding circuit 8000 carries out sampling/holding on the electric signal to provide the low speed signal LO. The frequency of the low speed signal LO is a beat frequency. In other words, the frequency is equal to the difference between the frequency of the signal Vin and the frequency of the optical pulse PF or the optical signal Pin. Accordingly, the change in the signal Vin can be represented by the low signal LO.
FIG. 4 shows the structure of an optical interferometer type optical modulator 9000. Unlike the optical modulator 2000 of FIG. 1, the optical modulator 9000 comprises a plate 9100 made of electrooptical effect crystal such as LiNbO.sub.3. The optical modulator 9000 further includes an input port 9000A, an output port 9000B, a division port 9000C, a combination port 9000D, and optical paths 9000E-1 and 9000E-2. The input port 9000 is formed on a side of the plate 9100 while the output port 9000B is formed on the opposite side thereof. The division port 9000C and the combination port 9000D are formed between the input port 9000A and the output port 9000B, wherein the optical paths 9000E-1 and 9000E-2 are formed in parallel with each other therebetween.
An electrode 9200A is formed along the optical path 9000E-1, an electrode 9200B is formed between the optical path 9000E-1 and the optical path 9000E-2, and an electrode 9200C is formed along the optical path 9000E-2. The signal Vin generated by the signal source 1000 is applied across the electrodes 9200A and 9200B, while both the electrodes 9200B and 9200C are grounded. The laser pulse source 3000 is connected to the input port 9000A, and the optical detector 6000 is connected to the output port 9000B.
The optical pulse PF generated by the laser pulse source 3000 Is fed into the input port 9000 A to be divided into two components at the division port 9000C. One component advances along the optical path 9000E-1, while the other component advances along the optical path 9000E-2. The former component changes in propagation velocity through the electric field formed by the signal Vin, while the velocity of the latter component remains unchanged. Hence, the components interfere with each other according to the change of the former component in the propagation velocity at the combination port 9000D. The light that is phase-modulated along the optical path 9000E-1 is combined at the combination port 9000D to be an amplitude modulation light by interference. Consequently, the amplitude modulation light, that is, an intensity modulation light is output from the output port 9000B.
FIG. 5 shows a relationship between the signal Vin and the ratio of the signal Pout to the signal Pin. Here, unlike the above bulk-type optical modulator accompanied by the polarizer, the optical signal PF is identical with the optical signal Pin. The relationship is given on assumption that the length of the optical path 9000E-1 and that of the optical path 9000E-2 are equivalent to each other. In the figure, with respect to the characteristic when the signal Vin is zero or near zero, Vin=0 provides the maximum ratio while both Vin&gt;0 and Vin&lt;0 provides other ratios smaller than the maximum ratio.
FIG. 6 shows another relationship between the signal Vin and the ratio of the signal Pout to the signal Pin. The relationship is given based on an assumption that the length of the optical path 9000E-1 differs from that of the optical path 9000E-2 by .lambda./4 where .lambda. denotes the wavelength of light. The characteristic around Vin=0 in FIG. 6 is widely and sharply linear similar to the relationship in FIG. 3 concerning the optical modulator 2000. Accordingly, this characteristic is more useful than that in FIG. 5.
As described above, the signal Vin can be sampled using either the bulk-type optical modulator 2000 or the interference-type optical modulator 9000. However, with respect to the characteristic of the signal Vin--the optical signal Pout, the linear range of the sine curve around Vin=0 is rather small. Therefore, it the amplitude of the signal Vin excesses the linear range, the optical signal Pout or the low speed signal Lo distorted, which disables accurate measurement of the high speed signal Vin using the low speed signal LO.